Metal detectors are used in a variety of applications, ranging from military and security uses such as detecting firearms or landmines to hobby uses such as searching for lost jewelry, coins, artifacts and other metal objects. When searching for buried metal objects, the searcher often must dig up the object for identification once it has been detected by the metal detector. If the object is not of particular interest, the searcher has needlessly invested the time and effort to dig up the object. In addition, needless damage is done to the terrain.
Especially for hobby users of metal detectors, ferrous metal objects, that is, objects made largely of iron or steel, are generally of less interest than are non-ferrous metal objects. Ferrous metal objects include items of little value or interest such as nails, staples, scrap iron, “tin” cans, steel parts of abandoned automobiles, or construction debris such as flashing. In contrast, non-ferrous metal objects tend to be of greater interest, although there are exceptions such as aluminum foil and pulltabs. Desirable objects for hobbyists include coins, composed of metals such as gold, silver, nickel or copper; jewelry made of gold, platinum or silver; bronze or brass artifacts such as antique cannons or ship's bells; or copper sheeting or wire which may be sold for salvage. The greater desirability of these types of objects may be due to their higher monetary value, their historical interest, or their physical attractiveness. Therefore metal detectors with improved operational features for identification of the composition of metal objects are desired by users.
There are two fundamentally different types of metal detectors generally in use at the present time, continuous wave and pulse types. The principle of operation of metal detectors of the continuous wave type involves the production and transmission of a substantially continuous time-varying, or alternating, electromagnetic signal which may consist of a single frequency or may have multiple frequency components, and the concurrent detection of a responsive time-varying electromagnetic signal that is induced by the transmitted electromagnetic signal in metal objects that are located near the detector. The pulse type detector emits brief bursts of electromagnetic radiation then detects a responsive signal from the metal object after the transmitted pulse ends.
Metal detectors may obtain information about the composition of metal objects by making use of the differing responses of various metal elements and alloys to the transmitted electromagnetic signal. Different metal substances, due to their diverse physical properties, cause different degrees of delay in the phase of the responsive electromagnetic signal, which may be determined by the detector using circuitry adapted for the purpose.
Various types of metals each have, at least to some extent, characteristic ranges of phase delays, although the phase delay as determined may also be affected by variables other than the material of composition such as shape and size of the object. While identification of the material composing a metal object does not unambiguously define what the object is, it does allow for objects unlikely to be of interest to be rejected and others to be identified as more likely of being worthy of recovery.
In the continuous wave type metal detector, analysis of a received signal is performed in the frequency domain. The phase delay angle of the received signal, relative to the transmitted signal or to a standard, consists of two electromagnetic components referred to as a resistive component and a reactive component. These two components are mathematically orthogonal, being disposed in a vector relationship at 90 degrees (π/2 radians) to each other. These two components sum by vector addition to yield the vector that expresses the angle of phase delay, therefore, the ratio of the two vectors expressing the magnitudes of the resistive and reactive components is equal to the arctangent of the phase delay angle. In the metal detector signal, it is traditional to assign the resistive value to the Y-axis and the reactive value to the X-axis when plotting the phase angle on Cartesian coordinates, so the phase delay angle is equal to the arctangent of resistive component magnitude divided by reactive component magnitude.
Thus, ferrous and non-ferrous metals produce responsive electromagnetic signals with differing characteristic properties of the resistive and reactive components which may be detected and used to provide the operator of the metal detector with information about the composition of a detected object. A characteristic of ferrous objects is to produce signals in response to the interrogating electromagnetic signal in which the polarities of the resistive and reactive components are opposite, while non-ferrous objects produce signals in which the polarities of the resistive and reactive components are the same. Furthermore, the magnitudes, i.e. the absolute values as well as the polarities, of the resistive and reactive components tend to have characteristic ranges for metals of a given type. It is known for metal detectors to use information of this type to characterize the type of metal composing a detected object.
The characterization can be used in different ways, for instance by reporting all metal objects along with information about their possible composition, or alternatively by only reporting metal objects whose composition has been pre-selected as being of a desirable type, or by providing a different type of indication for different compositions. The decisions as to how the detector will respond, and what types of metal compositions it will report to the user, may be made by the detector manufacturer and be preset at the factory, or alternatively the user may select and set criteria that define what phase delay angle values or ranges the metal detector will report or not report.
For example, U.S. Pat. No. 4,700,139 describes a metal detector that allows the user to electronically define a single excluded region of phase delay within which a target will not be reported, while targets whose phase delays, i.e., whose ratio of resistive and reactive components, are outside the excluded region are reported. U.S. Pat. No. 5,148,151 describes a detector into which the user can pre-program criteria for all regions of the phase delay scale that govern whether a detected object is reported (accepted) or not reported (rejected). U.S. Pat. No. 4,677,384 provides a discriminate circuit enabling the reporting of an object whose responsive electromagnetic signal falls within a certain narrow angle range of phase delay lying within a broader range of phase delay angles that would normally not be reported.
Despite this capability, in some cases the determination made by the metal detector about the material of composition of the detected object based on the phase delay angle can be incorrect. When a detector is preset to report only non-ferrous objects but then mistakenly reports a ferrous object, the user is mistakenly induced to expend time and effort digging up or otherwise securing the detected object only to find that it is not of the composition expected. Therefore, features that increase the accuracy and reliability of the identification of the metal of composition of an object made by the detector are desirable to users.
Accordingly, there is a need for an improved method and apparatus for detection of metal objects of non-ferrous composition with a reduced rate of false positive reports.